Cathode-ray tube



April 15, 1952 w. H. BUCHANAN CATHODE-RAY TUBE Filed NOV. 21, 1949 WILLIAM HUDSPITH BUCHANAN INVENTOR.

HIS ATTORNEY Patented Apr. 15, 1952 CATHODE-RAY TUBE William-.Hudspith Buchanan, Writtle, England,

assignor to Cinema-Television Limited, Lon don,England, a British company Application November 21, 1949, Serial No. 128,533 In Great Britain December 14, 1948 6 Claims. 1

This'invention relates to cathode-ray tubes including the type used in television receivers or the like, and more particularly this invention relates to a method and meansfor substantially eliminating the defocusing' effect upon the oathode-ray beam developed in cathode-ray tubes of the ions of residual gas within the tube envelope.

It is well known in the art that the ions of residual gases existin in'a cathode-ray tube affeet the trajectories of the cathode-ray beam developed therein. One effect of these ions is to move the axial position at which the beam comes into focus and thus de-focusthe beam. This defocusing effect of the ions on the cathode-ray beam is particularly noticeable incathode-ray reproducing tubes presently used in television receivers or the like. The reason for this is that in such reproducing tubes the cathode-ray is deflected over a relatively wide beam angle and a small change in the axial focal distance can produce a noticeable increase in the size of the illuminated spot formed by the beam as it impinges on the fluorescent screen contained in the tube.

It has been found that the ionization of the residual gases within a cathode-ray tube is caused mainly by the electron beamitself, and that the state of equilibrium of the ionization (as judged by its final effect on the focused spot) is reached a short time after any change in the excitation of the beam. Accordingly, any

change in the excitation of the cathode-ray beam gives rise to a change in theionization of the residual gases within the tube which, in turn, changes the axial position within the tube where the beam comes into focus. the cathode-ray beam is most noticeable at positions onthe screen where the excitation of the scanning beam changes to a dark area representation from a light area representation, that is when the excitation of thebeam changes from a minimum to a peak value. This change in excitation of the beam gives rise to increased ionization of the residual gases, and if the beam is adjusted to be in focus at the average gray level of excitationit-becomes de-focused when the excitation changes to' the dark area representation and remains de-focused until the excitation again changes to the gray level representation. The time taken for the cathode-ray beam to reach maximum de-focus upon change of excitation is of the order of a few micro-seconds, and is determined by the gas pressure existing in the particular tube. The lower the gas pressure the longer-the time taken for the beam to reach the maximum dc-focused condition.

This de-focusing of Many attempts have been made electrically to arrange the beam-focusing system of the cathode-ray tube in such a, manner that such changes in the excitation of the cathode-ray beam produce compensating changes in the focusing current and, thus, avoid the afore-described de-focusing effect. Such arrangements have been found unsatisfactory since it is difficult to adjust them properly, and also since it is impossible to construct such arrangement in a manner so that the compensating re-focusing effect takes place with the identical time lag as the de-focusing after each change in excitation of the cathode-ray beam.

The present invention provides a cathode-ray tube or the like in which auxiliary means independent of the scanning cathode-ray beam is incorpo-rated to ionize the residual gases in the tube. With such an arrangement, any change in the excitation of the scanning beam does not give rise to a material change in the ionization and the afore-described tie-focusing of the scanning beam is therefore substantially obviated.

It is, accordingly, an object of this invention to provide an improved method for substantially eliminating the de-focusing effect upon the cathode-ray beam developed in a cathode-ray tube of the ions of residual gas within the tube envelope.

Another object of this invention is to provide an improved cathode-ray tube .or the like in which changes in excitation of the cathode-ray beam therein have noxmaterial effect onthe focusing of the beam.

A further object of this invention is to provide an improved cathode-ray tube or-the like, in which auxiliary means is incorporated independent of the scanning cathode-ray beam therein to ionize 'theresidual gases in the cathode-ray tube, so that changes of excitation of the scanning beam do not give rise to a material changein the ionization and the axial focal point of the scanning beam is, therefore, not altered to any appreciable extent by such excitation changes.

Theieatures of this invention which are believed to be new are set forth with particularity in the appended claims. The invention itself, however, together With further objects and advantages thereof may best be understood by reference to the following description when taken in conjunction with the accompanying drawing, in which the single figure illustrates merely by way of example a cathode-ray tube constructed in accordance with one embodiment of the invention.

ing a scanning beam therein to excite the screen l3, comprises an electrically heated cathode I, a beam-modulating or exciting electrode '3 surrounding the cathode I, and an accelerating electrode 4. These electrodes are supported in well-known manner by connecting leads sealed in the re-entrant portion I6 of the tube.

The accelerating electrode 4 has a flange formed integrally therewith, and mounted on the side of this flange facing the screen 13 is a ring shaped cathode 1 for producing an auxiliary cathode beam in the tube surrounding the scanning beam. The cathode I is aligned with an annular focusing electrode 8 and an annular accelerating electrode 9, electrodes 8 and 9 being similarly supported by connecting leads sealed in the portion [6. The cathode-ray tube I is also provided with a usual focusing coil 14 and deflection yoke l5, of any known types.

The heater leads of cathodes l and I may be connected to any known type of energizing source. The leads of accelerating electrodes 4 and 9 may be connected to conventional directcurrent potential sources. The lead of the modulating-electrode 3 may be connected to any suitable modulating-potential source such as the video-amplifier of a television receiver.

.The focusing electrode 8, similarly, has its lead connected to a suitable direct-current potential source. Moreover, the focusing coil I4 is connected to an appropriate direct-current focusing source, and the deflection yoke I is connected to deflection systems, such as the sweep circuits of a television receiver.

The potential values of the various electrode potential sources are, preferably, such that the electrons in the auxiliary cathode-ray beam emitted from cathode 1 have a low velocity com-- pared with the electrons in the scanning beam originating at cathode I, and the auxiliary beam is utilized to ionize the residual gases in the tube. The velocity of the electrons in the auxiliary beam is sufficiently low so that these electrons do, not excite appreciably the fluorescent screen l3. In fact, when a metal-backed screen is used, the velocity of the electrons in the auxiliary beam is preferably such that these electrons do not penetrate the metal backing layer.

The ionizing effect of the low velocity electrons of the auxiliary beam has been found to be comparable with and even higher than the ionizing effect of the high-velocity electrons in the scanning beam. Moreover, these low-velocity electrons produce more ions per unit of current due to the longer paths they travel owing to their greater deflection in the magnetic focusing field of coil M. The arrangement is such that the low-velocity electrons ionize such a large portion of the available gas molecules readily be achieved. This may be accomplished by coupling the electrodes 8 or 9 to the previously-mentioned modulating source through any suitable phase-reversing circuit.

An example of the various potential values that may be applied to the cathode-ray tube I0 is as follows:

Cathode l, 0 volts Modulating electrode 3, (800 to 400) (exciting potential) Accelerating electrode 4, 50 kilovolts Cathode 1, 50 kilovolts Focusing electrode 8, 50 kilovolts Accelerating electrode 9, 50 kilovolts plus an accelerating potential from 50 to 5,000 volts The focusing coil l4 provides a magnetic focusing field for the scanning beam, and also acts to focus the auxiliary beam into an annular configuration surrounding the scanning beam and producing the required ionization. The deflection yoke l5 provides the usual deflection fields for the scanning beam, and acts to scan the scanning beam and the auxiliary beam over the fluorescent screen l3. As previously pointed out, the electrons in the auxiliary beam preferably have insufficient velocity to excite the screen l3 to any appreciable extent and may be ignored so far as scanning of the screen is concerned.

The focusing electrode 8 or the accelerating electrode 9 for the auxiliary beam instead of being maintained at a constant potential, may (as previously pointed out) be supplied with an exciting potential to modulate the auxiliary beam in an opposite sense to the modulation of the scanning beam. In this manner, the electrons in the auxiliary beam compensate for the ionizing action of the electrons in the scanning beam so that subsequentially constant although incomplete ionization is produced within the tube.

This invention provides, therefore, an improved cathode-ray tube in which auxiliary means is provided for ionizing the residual gases within the tube so that these gases do not have an adverse effect on the focusing of the cathoderay scanning beam developed therein.

Although one embodiment of a cathode-ray tube for carrying out the invention has been shown and described, modifications may be made, and it is intended in the appended claims to cover all such modifications as may fall within the true spirit and scope of the invention.

I claim:

1. A cathode-ray image-reproducing tube having residual gas contained therein comprising: an image-reproducing screen positioned at one end of said tube; an electron gun structure positioned at the opposite end of said tube for forming a cathode-ray beam in said tube and for directing said beam onto said screen; deflection elements for scanning said beam over said screen; a modulating electrode for modulating said beam in accordance with image intelligence; and an auxiliary electron source positioned at said opposite end of said tube for developing electrons in said tube in the region substantially surrounding the cathode-ray beam produced by said electron gun structure independent of said cathode-ray beam and having a low velocity relative to the electrons in said beam to produce ionization of said residual gas and prevent defocusing of said beam upon modulation thereof by said image intelligence.

2. A cathode-ray image-reproducing tube having residual gas contained therein comprising: an image-reproducing screen positioned at one end of said tube; a first electron source positioned at the opposite end of said tube for developing electrons within said tube; means associated with said first source for forming said electrons into a scanning cathode-ray beam and for directing said beam onto said screen; deflection elements for scanning said beam over said screen; a modulating electrode for exciting said beam in accordance with image intelligence; an auxiliary electron source positioned at said opposite end of said tube for-developing electrons in said tube in the region substantially surrounding said scanning cathode-ray beam independent of said cathode-ray beam to produce ionization of said residual gas; an accelerating electrode associated with said auxiliary source for imparting a velocity to said electrons from said auxiliary source that is low compared with the velocity of said electrons in said scanning cathode-ray beam; and a modulating electrode associated with said auxiliary source for modulating the electrons therefrom in accordance with said image intelligence but in an opposite sense to the modulation of said scanning beam to prevent defocusing of said scanning beam upon modulation thereof by said image intelligence.

3. A cathode-ray image-reproducing tube having residual gas contained therein comprising: an image-reproducing screen positioned at one end of said tube; a first electron-gun structure positioned at the opposite end of said tube ior forming a scanning cathode-ray beam in said tube and for directing said beam onto said screen; deflection elements for scanning said beam over said screen; a modulating electrode for exciting said beam in accordance with image intelligence; and a second electron gun structure positioned at said opposite end of'said tube for forming an auxiliary cathode-ray beam in said tube substantially in the region surrounding said scanning cathode-ray beam independent of said scanning cathode-ray beam and having a low velocity relative thereto to produce ionization of said residual gas and prevent defocusing of said scanning beam upon modulation thereof by said image intelligence.

4. A cathode-ray tube having residual gas contained therein comprising: a target; a first electron gun structure for forming a, scanning cathode-ray beam in said tube and for directing said beam onto said target; a modulating electrode for exciting said beam in accordance with intelligence; and an annular electron gun structure mounted coaxially with said first electron gun structure for forming an auxiliary cathoderay beam surrounding said scanning cathode-ray beam to produce ionization of said residual gas.

5. A cathode-ray tube having residual gas contained therein comprising: a target; a first electron source for developing electrons within said tube; means associated with said first source for forming said electrons into a scanning cathode-ray beam and for directing said beam onto said target; a modulating electrode for exciting said beam in accordance with intelligence; an annular auxiliary electron source mounted coaxially with said first electron source for developing electrons within said tube; an annular focusing electrode associated with said auxiliary electron source for forming said electrons from said auxiliary source into an auxiliary cathoderay beam surrounding said scanning beam to produce ionization of said residual gas; and an annular accelerating electrode for directing said auxiliary beam to said target with a velocity low compared with that of the electrons of said scanning beam.

6. A cathode-ray tube having residual gas contained therein comprising: a target; a first electron gun within said tube for developing a scanning cathode-ray beam and for directing said beam onto said target; a modulating electrode associated with said gun for exciting said beam in accordance with intelligence; an annular auxiliary electron gun mounted coaxially with said first-mentioned electron gun for developing an auxiliary cathode-ray beam surrounding said scanning beam, and an annular modulating electrode associated with said auxiliary gun for exciting said auxiliary beam in accordance with said intelligence but in an opposite sense to the excitation of said scanning beam to produce ionization of said residual gas to compensate for ionization of said gas by said scanning beam.

WILLIAM HUDSPITH BUCHANAN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,014,539 Stansbury Sept. 17', 1935 2,114,136 Batchelor Apr. 12, 1938 2,406,370 Hansen et a1. Apr. 27, 1946 2,457,175 Parker Dec. 28 1948 2,459,724 Selgin Jan. 18, 1949 2,504,231 Smith Apr. 18, 1950 

